Electric cable and manufacturing process thereof

ABSTRACT

An electric cable for power transmission at low voltage has at least one conductor and an insulating coating surrounding the at least one conductor. The insulating coating has at least two insulating layers. In a radial direction from the inside toward the outside of the electrical cable, the insulating coating has at least one insulating layer made of a non-expanded polymeric material and at least one insulating layer made of an expanded polymeric material.

The present invention relates to an electric cable with increasedflexibility and peeling-off properties.

Moreover, the present invention relates to an electric cable withincreased intelligibility of the marked indicia thereon.

In particular, the present invention relates to an electric cable forpower transmission at low voltage, preferably said electric cable beingsuitable for building wiring.

Furthermore, the present invention relates to a manufacturing process ofsaid electric cable.

In the present description, the expression “low voltage” means a voltageof less than about 1 kV, the expression “medium voltage means a voltageof between about 1 kV and about 30 kV, the expression “high voltage”means a voltage of between about 30 kV and about 220 kV, while theexpression “extra high voltage” means a voltage of greater than about220 kV.

Cables for power transmission at low voltage are generally provided witha metallic conductor which is surrounded by an insulating coatingadhering to said metallic conductor.

In the present description the expression “cable core” indicates astructure comprising at least one conductor and a respective electricinsulating coating arranged in a position radially external to saidconductor.

For the purposes of the present description, the expression “unipolarcable” means a cable provided with a single core as defined above, whilethe expression “multipolar cable” means a cable provided with at leastone pair of said cores. In greater detail, when the multipolar cable hasa number of cores equal to two, said cable is technically defined asbeing a “bipolar cable”, if there are three cores, said cable is knownas a “tripolar cable”, and so on.

In a position radially external to said insulating coating, cables forpower transmission at low voltage can be provided with an outerpolymeric sheath having the function of mechanically protecting thecables from the external environment, e.g. from any impact and/orabrasion that might lead to cable cracks or ruptures formation. In amultipolar type configuration, in the case said outer protectivepolymeric sheath is present, the multipolar cable is provided with acommon sheath surrounding the cable cores as defined above.

Document. U.S. Pat. No. 4,789,589 discloses an electric cable providedwith a double layer insulating coating arranged upon a conductiveelement, said double layer comprising an inner layer of a polyolefincompound and of a cellular construction, and an outer layer of a givenmaximum thickness and of a non-cured and non-curable polyolefin basecompound having a solid (i.e. non-expanded) construction, said basecompound including a material compatible with the polyolefin of theinner layer. Preferably, the polyolefin of the outer layer is polyvinylchloride. Document U.S. Pat. No. 4,789,589 refers to the problem ofseparation of the thin uncured outer layer from the inner layer duringthe manufacturing process and/or during installation of the cable. TheApplicant observed that such a solution can not correctly work since theinner insulating layer, due to its expanded state, presentsdiscontinuities (i.e., voids within the polymeric material, said voidsbeing filled with air or gas) in the space surrounding the conductor,i.e. in the space where the electrical field is the most relevant.

Data communication cables provided with a double layer insulatingcoating comprising an inner layer of an expanded polyolefin compound andan outer layer of a non-expanded polyolefin base compound are describedin documents CA-952,991 and U.S. Pat. No. 5,841,072.

Document JP 90-35544 discloses an electric low voltage distributioncable comprising a pair of twisted insulated conductors between which afoaming material is arranged. A sheathing material is also provided forcovering the twisted cable cores. The foaming and the insulatingmaterial can be polyvinyl chloride.

Document U.S. Pat. No. 3,013,109 relates to non-metallic sheathed cablesfor building wiring comprising a protective sheath made of expandedcellular organic material. According to said document, the insulation ismade of a dense solid material (e.g. a dense semi-rigid polyvinylchloride) while the outer protective sheath is composed essentially of atough, flexible resinous plastic material such as polyvinyl chloride inexpanded cellular form. Furthermore, document U.S. Pat. No. 3,013,109states that the outer protective sheath is distinct from the insulationand is slippable or slidable thereon with the result that the cable canbe readily bent without distortion in the plane of the conductors and,after being deliberately bent at a selected angle, the cable maintainsthe bent shape indefinitely.

Document JP 11-203941 discloses a method for manufacturing a cableprovided with an insulating coating obtained from a resin compositionmainly comprising vinyl chloride resin, and with an expanded sheathlayer obtained from a composition mainly comprising vinyl chloride resininto which a foaming agent has been compounded.

Document WO 98/52197, in the name of the Applicant, discloses the use ofa layer made of expanded polymeric material of a suitable thickness tobe applied in a position radially external to the aforesaid cable core.According to said document, the expanded layer confers the cable highresistance to accidental impacts which might be suffered by the latterduring the steps of cable transport or laying. Said impacts are verydangerous for the cable since they can cause considerable damage to thecable structure (for example deformation of the insulating layer,detachment of the cable layers) determining, for example, changes in theelectrical gradient of the insulating layer with a consequent reductionin its insulating capacity. Said expanded layer is preferably located ina position immediately below the outer polymeric sheath of the cableand, being able of endowing the cable with high impact strength, itmakes possible to eliminate any traditional, generally metallic,protective armours. In order to confer the cable the desired impactresistance, the expanded layer is obtained from a polymeric materialwhich, before expansion, has a flexural modulus at room temperature(measured according to ASTM Standard D790) of at least 200 MPa.

Document U.S. Pat. No. 3,936,591 discloses a non metallic-sheathed cablefor building wiring comprising a wall of expanded polyvinyl chlorideinsulation surrounding each cable conductor and a thin-walled tubularjacket of polyvinyl chloride surrounding the totality of insulatedconductors, said jacket conferring mechanical protection to the nonmetallic-sheathed cable.

Generally, a cable for building wiring is installed within the walls ofa building and the installation process requires that the cable passesthrough walls restrictions or, more frequently, that the cable is pulledthrough conduits, wherein the cable is permanently confined.

In order to be correctly installed with simple and quick operations, acable for building wiring needs to be particularly flexible so that itcan be inserted into the wall passages and/or wall conduits and followthe bends of the installation path without being damaged.

During customer installation, due to the tortuosity of the installationpath and to friction during the pulling operation, the cables forbuilding wiring are generally subjected to tearing or scraping againstrough edges and/or surfaces.

The Applicant has perceived that an increased flexibility of an electriccable for building wiring can allow to reduce the damages caused by saidtearing or scraping actions.

Furthermore, a very important aspect which is required to be satisfiedby a cable for building wiring is a simple and quick peeling-off of thecable. In the present description the term “peeling-off of a cable” isused to indicate the removal of all the cable layers which are radiallyexternal to the conductor so that it results uncoated and can beelectrically connected to a conductor of a further cable or to anelectrical apparatus.

The peeling-off property of a cable for building wiring is a widely feltrequest of the market since the peeling-off of a cable is an operationwhich is manually performed by the technical staff. For this reason,said operation is required to be easy and quick to be performed by theoperator, taking also into account that it is frequently carried out innarrow spaces and rather uncomfortable conditions.

The Applicant has found that it is possible to improve the flexibilityand the peeling-off property of a cable for building wiring by providingthe latter with an insulating coating which has an expanded portion.Preferably, said expanded portion of the insulating coating is acircumferentially extended layer.

In particular, the Applicant has found that said expanded insulatinglayer has to be preferably positioned not adhering to the cableconductor, i.e. not directly contacting the cable conductor. In otherwords, the Applicant has found to provide the cable with an insulatingcoating comprising at least two insulating layers so that, in a radialdirection from the inside towards the outside of the cable, theinsulating coating comprises at least one insulating layer made of anon-expanded polymeric material and at least one insulating layer madeof an expanded polymeric material.

The Applicant has found that the presence of said expanded insulatinglayer in a position radially external to the non-expanded insulatinglayer is particularly advantageous in respect of the peeling-offproperty of the cable and the flexibility thereof.

The Applicant has found that the expanded insulating layer exerts on theconductor a ringing force in the radial direction which is lower thanthe ringing force exterted on the conductor by a non-expanded insulatinglayer. For this reason, the force to be exterted by the operator to peeloff the cable insulating coating is remarkably reduced and thepeeling-off property of the cable is favourably increased.

Moreover, by providing the cable with an expanded insulating layer theflexibility of the cable is advantageously increased with favourableresults in the installation process thereof.

In a first aspect the present invention relates to an electric cablecomprising a conductor and an insulating coating surrounding saidconductor, said insulating coating having a predetermined thickness andcomprising at least two insulating layers, characterized in that, in aradial direction from the inside towards the outside of said electricalcable, said insulating layers comprise at least one insulating layermade of a non-expanded polymeric material and at least one insulatinglayer made of an expanded polymeric material, said at least oneinsulating layer made of a non-expanded polymeric material beingintegral with said at least one insulating layer made of an expandedpolymeric material.

According to the present invention, the predetermined thickness of saidinsulating coating is such that said insulating coating confers to thecable the electrical insulating features required by the use it isprovided for (e.g defined by relevant Standards). In other words, in thecase the cable is provided with a sheath radially external to saidinsulating coating, said sheath does not contribute to obtain saidrequired electrical insulating features, the required values of thelatters being guaranteed by the insulating coating of predeterminedthickness.

According to the present invention, the thickness of said insulatinglayer made of a non-expanded polymeric material is at least half of saidpredetermined thickness of said insulating coating. Preferably, thethickness of said insulating layer made of a non-expanded polymericmaterial is not lower than 70% of the predetermined thickness of saidinsulating coating, more preferably the thickness of said insulatinglayer made of a non-expanded polymeric material is not lower than 85% ofthe predetermined thickness of said insulating coating.

According to the present invention, the expanded insulating layer isintegral with the non-expanded insulating layer so that said layers arebonded together to form the cable insulating coating. In the presentdescription, the term “integral” is used to indicate that a unitarystructure was obtained. Therefore, in the present description theexpression “the expanded insulating layer is integral with thenon-expanded insulating layer” means that “the expanded insulating layeris formed as a unit with the non-expanded insulating layer”. In otherwords, this means that the expanded and non-expanded insulating layersare bonded together and, once produced, they can not be separatedwithout a cutting means or the like, for instance they can not beseparated by applying a traction force thereto or by heating.

Processes for obtaining a unitary structure as defined hereinabove, saidunitary structure consisting of a first layer and a second layer, canbe, for instance: a) a co-extrusion process of said firs-t and secondlayers; b) the “tandem” technique according to which the extruder ofsaid first layer and the extruder of said second layer are arranged inseries. Alternative embodiments of said abovementioned processes caninclude the step of application of a suitable adhesive layer betweensaid first and second layers, for instance by means of said co-extrusionprocess or said “tandem” technique. Particularly preferred is theco-extrusion technique so that the expanded insulating layer of thecable insulating coating is co-extruded with the non-expanded insulatinglayer of said cable insulating coating.

Preferably, the cable according to the present invention is not providedwith a sheath layer, unless a mechanical protective layer againstaccidental impacts or special abrasion resistance during theinstallation process thereof is required. In the present description,the term “cable sheath layer” is intended to identify a protective outerlayer of the cable having the function of protecting the latter fromaccidental impacts or abrasion. From the foregoing, according to theterm mentioned above, the cable sheath layer is not required to providethe cable with specific electrical insulating properties.

Moreover, the Applicant has found that the cable according to thepresent invention, thanks to the presence of the expanded insulatinglayer, can be easily and effectively marked. Generally a cable, such asa cable for building wiring, needs to be marked in order to properlyidentify the cable. Indicia which are generally provided to the externalsurface of a cable are, for example, the trade name, the name of themanufacturer, the Standard the cable is in accordance with, theproduction year. Generally, said indicia are provided every 0.5 m-1.0 mof cable length and it is important that the marked letters can beintelligible and simply recognizable by the operator. The Applicant hasfound that the expanded insulating layer increases the intelligibilityof the marked letters. In fact, the presence of said expanded insulatinglayer allows the marked letters to stand out from the cable surface moreclearly with respect to the case in which the expanded layer is notpresent.

Furthermore, the expanded insulating layer of the cable according to thepresent invention advantageously decreases the total weight of the cableso that its installation and transportation is easier and its costs canbe remarkably reduced.

In a further aspect of the present invention, the cable insulatingcoating according to the present invention comprises three insulatinglayers. In a radial direction from the inside towards the outside of theelectric cable, said insulating coating comprises: a) an innerinsulating layer made of a non-expanded polymeric material; b) anintermediate non continuous insulating layer made of an expandedpolymeric material; c) a continuous external insulating layer made of anexpanded polymeric material.

In a further aspect of the present invention, the cable insulatingcoating according to the present invention comprises, in a radialdirection from the inside towards the outside of the electric cable: a)an inner insulating layer made of a non-expanded polymeric material; b)an intermediate insulating layer made of an expanded polymeric material;c) an external insulating layer made of a non-expanded polymericmaterial. Preferably, said inner insulating layer and said externalinsulating layer are made of the same polymeric material.

The Applicant has found that, even though the expanded insulating layeris an intermediate layer and is not an external insulating layer, themarkability of the cable and the peeling-off property thereof arefavourably increased by the presence of said expanded layer.

In a further aspect the present invention relates to a process formanufacturing an electric cable comprising a conductor and an insulatingcoating surrounding said conductor, said insulating coating comprising,in a radial direction from the inside towards the outside of saidelectrical cable, at least one insulating layer made of a non-expandedpolymeric material and at least one insulating layer made of an expandedpolymeric material, said process comprising the steps of: a) feedingsaid at least one conductor to an extruding machine; b) depositing byco-extrusion: a non-expandable polymeric material in a position radiallyexternal to said at least one conductor so as to form said at least oneinsulating layer made of a non-expanded polymeric material; and anexpandable polymeric material in a position radially external to said atleast one insulating layer made of a non-expanded polymeric material soas to form -said at least one insulating layer made of an expandedpolymeric material; c) expanding said expandable polymeric materialduring said step of depositing by co-extrusion.

Further characteristics and advantages will become clearer in the lightof the following description of some preferred embodiments of thepresent invention.

The following description makes reference to the accompanying drawings,in which:

FIG. 1 shows a cross right section of an example of a cable according tothe present invention;

FIG. 2 shows a cross right section of an example of a further embodimentof a cable according to the present invention, and

FIG. 3 shows a cross right section of an example of a further embodimentof the cable of FIG. 2.

In the following of the present description, the term “expandedpolymeric material” means a polymeric material with a predeterminedpercentage of “free” space inside the material, i.e. a space notoccupied by the polymeric material, but by gas or air.

In general, said percentage of free space in an expanded polymer isexpressed by the so-called “expansion degree” (G), defined as follows:G=(d ₀ /d _(e)−1)*100where d₀ denotes the density of the unexpanded polymer and d_(e) denotesthe apparent density measured on the expanded polymer.

The expanded polymeric material of the expanded insulating layercomprises at least one expandable polymer. If necessary said polymer,after expansion, can be crosslinked, as described in the following ofthe present description.

Said expandable polymer can be selected from the group comprising:polyolefins, copolymers of various olefins, olefins/unsaturated esterscopolymers, polyesters, and mixtures thereof. Examples of suitablepolymers are: polyethylene (PE), in particular low-density PE (LDPE),medium-density PE (MDPE), high-density PE (HDPE) and linear low-densityPE (LLDPE); polypropylene (PP); ethylene-propylene elastomericcopolymers (EPM) or ethylene-propylene-diene terpolymers (EPDM); naturalrubber; butyl rubber; ethylene/vinyl ester copolymers, for exampleethylene/vinyl acetate (EVA); ethylene/acrylate copolymers;ethylene/α-olefin thermoplastic copolymers; polystyrene;acrylonitrile-butadiene-styrene resins (ABS); halogenated polymers, inparticular polyvinyl chloride (PVC); polyurethane (PUR); polyamides;aromatic polyesters; and their copolymers or mechanical blends.

Polyvinyl chloride is particularly preferred.

In the attached figures, similar or identical components have been giventhe same reference signs.

FIG. 1 shows the cross section of a first embodiment of a cable 10 forpower transmission at low voltage according to the present invention.

Cable 10 is of the unipolar type and comprises a conductor 1 and aninsulating coating 2 comprising two insulating layers 3, 4. In details,according to the embodiment shown in FIG. 1, the insulating coating 2comprises a first inner insulating layer 3 surrounding the conductor 1and adhering thereto, and a second insulating layer 4 which is coaxialwith and external to said inner insulating layer 3. The inner insulatinglayer 3 is non-expanded while the external insulating layer 4 is made ofan expanded polymeric composition having electrical insulatingproperties, said polymeric composition comprising at least oneexpandable polymer chosen from the group mentioned above.

The expansion degree of the external expanded insulating layer 4 isgenerally between 2% and 500%, preferably between 5% and 200%, morepreferably between 10% and 50%. As explained in the following of thepresent description, the expansion of the polymeric base of saidexternal insulating layer 4 is carried out during the extrusion step andcan be effected either chemically or physically. An expansion degreebetween 2% and 100% can be obtained by means of an. expansion of thechemical type. On the contrary, an expansion of the physical type canproduce a very high expansion degree (i.e., equal to 500%), but it ismore expensive than the chemical type. For the purposes of the presentinvention it is considered to be expanded a layer whose polymeric basehas an expansion degree not lower than 2%.

Furthermore, according to the embodiment of FIG. 1, the inner insulatinglayer 3 is made of a non-expanded polymeric composition havingelectrical insulating properties, said polymeric composition comprisingat least one polymer chosen, for example, from: polyolefins(homopolymers or copolymers of various olefins), ethylenicallyunsaturated olefin/ester copolymers, polyvinyl chloride (PVC),polyesters, polyethers, polyether/polyester copolymers, and blendsthereof. Examples of such polymers are: polyethylene (PE), in particularlinear low-density PE (LLDPE); polypropylene (PP); propylene/ethylenethermoplastic copolymers; ethylene/propylene rubbers (EPR) orethylene/propylene/diene rubbers (EPDM); natural rubbers; butyl rubbers;ethylene/vinyl acetate (EVA) copolymers; ethylene/methyl acrylate (EMA)copolymers; ethylene/ethyl acrylate (EEA) copolymers; ethylene/butylacrylate (EBA) copolymers; ethylene/a-olefin copolymers, and the like.

Polyvinyl chloride is particularly preferred.

Preferably, the insulating layers 3, 4 of the insulating coating 2 aremade of the same base polymer.

Preferably, said base polymer is polyvinyl chloride (PVC).

Except for the expanding agent, preferably the polymeric compositions ofthe non-expanded insulating layer and of the expanded insulating layerhave the same recipe ingredients.

For a conductor of a given cross section, Italian Standard CEI-UNEL35752 (2nd Edition—February 1990) sets a predetermined average thicknessof the insulating coating to be provided to the cable so that, at apredetermined temperature, a minimum electrical resistance of saidinsulating coating needs to be guaranteed. For example, for a singleconductor having a cross section of about 1 m², said Italian Standardrequires an average thickness of the insulating coating of about 0.7 mmin order to obtain, at 70° C., a minimum electrical resistance of saidinsulating coating of about 0.095 MOhm*km. For example, for a singleconductor having a cross section of about 10 m², said Italian Standardrequires an average thickness of the insulating coating of about 1.0 mmin order to obtain, at 70° C., a minimum electrical resistance of saidinsulating coating of about 1.91 MOhm*km.

Therefore, according to the present invention, the minimum averagethickness of the insulating coating of an electrical cable ispredetermined so that the required electric insulating properties arecompatible with the Standards (e.g. Italian Standard CEI-UNEL 35752 orany other Standard equivalent thereto) and are satisfied by saidinsulating coating.

For instance, the predetermined thickness of the insulating coating issuch that, at 70° C., the minimum value of the electric resistance ofsaid insulating coating is greater than 0.024.MOhm*km.

For instance, the minimum average thickness of the insulating coating ofthe electrical cable is not greater than 2.5 mm.

Furthermore, according to the present invention, the insulating constantk_(i) of the electrical insulating layers 3, 4 is such that the requiredelectric insulating properties are compatible with the Standards (e.g.Italian Standard CEI 20-11 or other equivalent Standards). For instance,the electrical insulating layers 3, 4 have an insulating constant k_(i)greater than 750 MOhm*km at 20° C. For instance, said insulatingconstant k_(i) is greater than 0.3 MOhm*km at 70° C.

According to the present invention, in order to confer to the externalinsulating layer a suitable mechanical resistance without decreasing theflexibility of the cable, the expanded polymeric material of theexternal insulating layer is obtained from a polymeric material that,before expansion, has a flexural modulus at room temperature, measuredaccording to ASTM standard D790-86, comprised between 20 MPa and 600MPa. Preferably, said flexural modulus at room temperature is notgreater than 200 MPa, more preferably it is comprised 25 between 20 MPaand 200 MPa, even more preferably it is comprised between 10 MPa and 150MPa.

Preferably, the external insulating layer 4 has a thickness comprisedbetween 0.05 mm and 1.00 mm, more preferably between 0.10 mm and 0.50mm.

FIG. 2 shows the cross section of a further embodiment of a cable 20 forpower transmission at low voltage according to the present invention.

Cable 20 is of the unipolar type and comprises a conductor 1 surroundedby a multilayer insulating coating 21. In details, according to saidembodiment the insulating coating 21 comprises: an inner insulatinglayer 3 surrounding the conductor 1 and adhering thereto; an externalinsulating layer 4 coaxial with said inner insulating layer 3; and anintermediate insulating layer 5 which is interposed between said innerinsulating layer 3 and said external insulating layer 4.

According to the embodiment shown in FIG. 2, the intermediate insulatinglayer 5 is circumferentially non-continuous in the cross section.Preferably, said intermediate insulating layer 5 presents at least oneinterruption. Even more preferably, said interruption is located alongthe external profile of the inner insulating layer 3. Alternatively,said interruption is located in proximity of the external profile of theinner insulating layer 3.

Preferably, said circumferentially non-continuous intermediateinsulating layer 5 comprises at least one sector, i.e. a portion, whichis substantially semicircular in shape (e.g. a portion which islenticular in shape).

According to the embodiment shown in FIG. 2, the semicircular sectorsare in the number of four and are obtained within the inner insulatinglayer 3.

Preferably, the ratio between: a) the sum of the lengths of the arcs 5 adefined by said sectors on the circumference of the external insulatinglayer 4, and b) the circumference of the external insulating layer 4itself, is greater than or equal to 0.5, more preferably greater than orequal to 0.7. Preferably, said ratio is lower than or equal to 1, morepreferably lower than or equal to 0.9.

According to a further embodiment (not shown), the semicircular sectorsare obtained within the external insulating layer 4.

The particular configuration shown in FIG. 2 can be advantageouslyemployed to rapidly and easily perform the change of colour of the cableto be produced. Generally, a cable for building wiring is suitablycoloured in order to simply distinguish one cable from another duringinstallation and/or use. The manufacturing of a cable having theconfiguration shown in FIG. 2 is carried out by providing the extrusionapparatus with a flow control device which is able to modify the flowpassage of the polymeric composition so that the polymeric material,which is used to form the external insulating layer, is successivelyused to form the intermediate insulating layer, and viceversa. In otherwords, said control device allows the flows of the polymeric materialsof the external insulating layer and of the intermediate insulatinglayer to be exchanged so that the change of the cable colour can beperformed easily and in a short length of the cable so that the scrapsare remarkably reduced.

According to the embodiment of FIG. 2, the inner insulating layer 3 isnon-expanded while the intermediate insulating layer 5 and the externalinsulating layer 4 are expanded.

FIG. 3 shows the cross section of a further embodiment of a cable 30 forpower transmission at low voltage according to the present invention.

Cable 30 differs from cable 20 of FIG. 2 in that the external insulatinglayer 4 of the insulating coating 31 is non-expanded. In details, theinsulating coating 31 comprises non-expanded inner and externalinsulating layers 3, 4 and an expanded intermediate insulating layer 5.

According to a further embodiment (not shown), the intermediateinsulating layer is a layer which is circumferentially continuous, inthe cross section. Preferably said circumferentially continuousintermediate insulating layer uniformly surrounds the whole externalprofile of the inner insulating layer.

The figures illustrated above show only some of the possible embodimentsof cables in which the present invention can be advantageously employed.Therefore, any suitable modifications can be made to the embodimentsmentioned above such as, for example, the use of cables of themultipolar type or conductors of sectorial cross section.

With regard to the manufacturing process of a cable according to thepresent invention, the main steps characterizing the aforesaid processin the case when the unipolar cable of FIG. 1 has to be produced arepresented in the following. However, the teaching given hereinbelow forthe manufacturing of a unipolar cable can be used also in the case amultipolar cable has to be produced.

The conductor 1, unwound from a suitable reel, is introduced into anextrusion apparatus which is suitable for providing the conductor 1 withthe insulating coating 2.

According to the embodiment of FIG. 1, the insulating coating 2comprises an inner insulating layer 3 which is non-expanded and anexternal insulating layer 4 which is expanded.

The expansion of the polymeric base of said external insulating layer 4is carried out during the extrusion step of the latter and can beeffected either chemically or physically. In the first case, theexpansion is effected by adding to the polymeric composition a suitableexpanding agent which is able of evolving a gas under predeterminedpressure and temperature conditions, i.e the pressure and temperatureconditions of the extruder head. In the second case, the expansion iseffected by injecting a gas at high pressure directly into the barrel ofthe extruder.

Preferably, according to the present invention, the insulating layers 3,4 of the insulating coating 2 are applied by co-extrusion so that theexpanded insulating layer 4 becomes integral with the non-expandedinsulating layer 3, said layers becoming bonded together to form thecable insulating coating 2.

In the case the expansion is chemically effected, examples of suitableexpanding agents are: azodicarbamide, paratoluene sulphonylhydrazide,mixtures of organic acids (e.g. citric acid) with carbonates and/orbicarbonates (e.g. sodium bicarbonate), and the like.

In the case the expansion is physically effected, examples of gases thatcan be injected at high pressure into the extruder barrel are: nitrogen,carbon dioxide, air, low-boiling hydrocarbons, e.g. propane or butane,halogenated hydrocarbons, e.g. methylene chloride,trichlorofluoromethane, 1-chloro-1,1-difluoroethane, and the like ortheir mixtures.

Preferably, the die of the extruder head has a diameter slightly lowerthan the final diameter of the cable with expanded covering that it hasto be obtained, in such a way that expansion of the polymer outside theextruder results in attainment of the desired diameter.

It has been observed that, in the same extrusion conditions (such asrotary speed of the screw, speed of the extrusion line, diameter of theextruder head) one of the process variables having most influence on theexpansion degree is the extrusion temperature. In general, a sufficientexpansion degree can be obtained at temperatures greater than 130° C.The extrusion temperature is preferably at least 140° C., morepreferably about 180° C. Normally, an increase in extrusion temperaturecorresponds to a higher expansion degree.

Furthermore, it is possible to control the expansion degree of thepolymer to some extent by acting upon the cooling rate. In fact, bydelaying or by suitably speeding up the cooling of the polymer formingthe expanded covering as it leaves the extruder, it is possible toincrease or decrease the expansion degree of said polymer. This can bemade, for example, by varying the flow rate of a cooling fluid (e.g.water) in a cooler positioned downstream of the extruder head.

Furthermore, the expanded polymeric material of the cable insulatinglayer of the insulating coating can undergo a cross-linking process.Cross-linking is effected, after the steps of extrusion and expansion,according to known techniques, in particular by heating in the presenceof a radical initiator, for example an organic peroxide such as dicumylperoxide. Alternatively, cross-linking can be effected using silanes,i.e. by using a polymer belonging to the group mentioned above, inparticular a polyolefin, to which are grafted, covalently, silane unitscomprising at least one hydrolysable group, for example trialkoxysilanegroups, in particular trimethoxysilane. Grafting of the silane units tothe polyolefin backbone can be made by a radical reaction with silanecompounds, for example methyltriethoxysilane, dimethyldiethoxysilane,vinyldimethoxysilane, and the like. Cross-linking is effected in thepresence of water and of a cross-linking catalyst, for example anorganic titanate or a metallic carboxylate. Dibutyltin dilaurate (DBTL)is especially preferred.

For further description of the invention, some illustrative examples aregiven below.

EXAMPLE 1

A first polymeric mix was prepared suitable for making the innerinsulating layer of a cable insulating coating.

The composition of said mix is shown in Table 1 (expressed in parts byweight per 100 parts by weight of base polymer, or phr).

Except for the plasticizing agent, the components of said firstpolymeric mix were firstly mixed in a closed mixer working at a constanttemperature of about 120° C. and achieving a suitable vacuum degree(i.e., a maximum residual pressure of about 100 mmHg). Successively,e.g. 10 sec after the introduction of the mix components, theplasticizing agent was introduced into said mixer. The polymeric mix,discharged at a temperature of about 120° C., was cooled at atemperature of about 70° C. and fed into an extruder. Thus, theextrudate was successively submitted to a pelletizing operation. TABLE 1PVC K70 Resin 100 (e.g. Evipol SH7020 ® produced by EVC) Antimonytrioxide 0.75 Calcium carbonate 60 Bisphenol A 0.62 Stabilizing agent 4Plasticizing agent 38 Mineral charge 2.5

Some samples in the form of plates were obtained from the pelletsmentioned above in order to carry out mechanical measures.

The flexural modulus of the polymeric material, before expansion, wasmeasured at room temperature (20° C.) according to ASTM standard D790and a value of 144 MPa was obtained.

Ultimate tensile stress was measured according to Standard IEC 60811 1-1(2^(nd) Edition, 1985) and a value of 16.8 MPa was obtained. Accordingto said Standard, the ultimate tensile stress of an insulating compoundis required to be not lower than 12.5 MPa, while the ultimate tensilestress of a sheathing compound is required to be not lower than 10 MPa.

Ultimate elongation was measured according to Standard IEC 60811 1-1 anda value of 250% was obtained.

EXAMPLE 2

A second polymeric mix was prepared suitable for making the externalinsulating layer of a cable insulating coating.

The composition of said mix is shown in Table 2 (expressed in parts byweight per 100 parts by weight of base polymer, or phr).

The components of said second polymeric mix were subjected to processsteps analogous to those described in Example 1. TABLE 2 PVC K70 Resin100 (e.g. Evipol SH7020 ® produced by EVC) Antimony trioxide 3 Calciumcarbonate 100 Bisphenol A 0.2 Stabilizing agent 8 Plasticizing agent 40Chlorinated paraffin 18

Some samples in the form of plates were obtained from said pellets inorder to carry out mechanical properties measures.

The flexural modulus of the polymeric material, before expansion, wasmeasured at room temperature (20° C.) according to ASTM standard D790and a value of 32.7 MPa was obtained.

Ultimate tensile stress was measured according to Standard IEC 60811 1-1and a value of 14 MPa was obtained.

Ultimate elongation was measured according to Standard IEC 60811 1-1 anda value of 320% was obtained.

EXAMPLE 3

In order to obtain during the extrusion process the expansion of theexternal insulating layer, a master-batch of the second polymericcomposition and of the expanding agent was prepared. The master-bach isreported in Table 3 hereinbelow (expressed in parts by weight—%wt).TABLE 3 Second polymeric composition 60% wt LAGOCELL20 ® 20% wt(expanding agent) LAGOCELLBO20 ® 20% wt (expanding agent)

EXAMPLE 4

Production of a low-voltage cable was undertaken according to the cabledesign shown in FIG. 1.

The cable conductor 1 was made of copper and had a cross section ofabout 1.5 m².

The cable conductor was provided with an insulating coating 2 consistingof an inner insulating layer 3 and an external insulating layer 4. Theinner insulating layer 3 and the external insulating layer were obtainedby co-extrusion providing the extrusion apparatus with a double layerextrusion head. The inner insulating layer was obtained by introducingthe first polymeric composition reported in Table 1 into a 120 mmsingle-screw extruder in 25 D configuration, with rotary speed of thescrew of about 20.3 rev/min. The external insulating layer was obtainedby introducing the second polymeric composition reported in Table 2together with 1.2% by weight of the master-batch reported in Table 3 and1% by weight of colouring agent (Polyone 3050 BK30® produced by Polyone)into a 120 mm single-screw extruder in 25 D configuration, with rotaryspeed of the screw of about 45 rev/min.

The thickness of the inner insulating layer was about 0.6 mm. Thethickness of the expanded external insulating layer was about 0.1 mm.Therefore, the overall thickness of the insulating coating was about 0.7mm in accordance with Italian Standard CEI-UNEL 35752 (2ndEdition—February 1990).

The speed line was about 570 m/min and the cable diameter was comprisedbetween 2.88 mm and 2.91 mm.

Tables 4 and 5 hereinbelow show the thermal profiles of the extruders ofsaid insulating layers and of the extrusion head of the extrusionapparatus, the latter being devided into a plurality of zonesidentifying distinct portion of the extruder along its longitudinalaxis. TABLE 4 Zone of the extruder of the inner insulating layerTemperature (° C.) Zone 1 125 Zone 2 145 Zone 3 145 Zone 4 145 Zone 5145 Neck 140 Die 150

TABLE 5 Zone of the extruder of the external insulating layerTemperature (° C.) Zone 1 130 Zone 2 150 Zone 3 160 Zone 4 155 Neck 150Die 150

The material of the insulating coating had a final density of 1.43 kg/land a expansion degree of 5%. The expansion degree of the externalinsulating layer alone was of about 30%.

The cable was successively cooled in water and finally wound on astorage reel.

Mechanical Properties

A sample of the cable produced according to the procedure described inExample 4 was tested in order to measure the most relevant mechanicalproperties of the cable.

The ultimate tensile stress, measured according to Standard IEC 608111-1 mentioned above, was of about 15 MPa, while the ultimate elongation,measured according to the Standard mentioned above, was of about of205%.

From a manual handling of the cable sample, the Applicant has detectedthat the flexibility thereof was sensibly improved.

Electrical Properties

The insulating constant k_(i) was measured according to Italian StandardCEI 20-11 mentioned above and values of about 750 MOhm*km at 20° C. andof about 0.7 MOhm*km at 70° C. were obtained.

Peeling-Off Property

A sample of the cable was tested by measuring the load (KN) necessaryfor extracting the insulating coating from the conductor.

The test was carried out as follows. A cable sample of about 180 mm inlength was provided so that a first end portion of the cable of about 50mm in length and a second end portion of the cable of about 80 mm inlength were prepared devoid of the insulating coating. Therefore, thecable consisted of an insulated central portion of about 50 mm in lengthand of first and second end portions made of the conductor only. Acylinder, provided with a longitudinal hole of a diameter correspondingto the external diameter of the insulated cable, was employed in orderto contain the cable sample. In details, the cable sample was insertedinto the longitudinal hole of the cylinder so that the whole first endportion of the cable came out from the cylinder while the centralportion and the second end portion of the cable were positioned insidethe cylinder. Since the external diameter of the insulated portion ofthe cable was substantially equal to the diameter of the hole of thecylinder, the latter was maintained in this position thanks to thefriction action between the walls of the hole and the insulated portionof the cable. A dynamometer was used to carry out the test, saiddynamometer being provided with upper and lower clamps. In details, thedynamometer fixing upper clamps were associated to the cable conductorof the first end portion while the dynamometer movable fixing lowerclamps were associated to the lower end of the cylinder so that thelatter could be moved downwardly (i.e. in the direction of thelongitudinal hole). The test was stopped when the cylinder was displaceddownwardly of a length equal to the length of the cable insulatedportion (i.e., about 50 mm) and the force necessary to obtain saiddisplacement (i.e., the force necessary to remove the insulating coatingfrom the conductor) was measured. The measured value was of about 0.025KN.

Marking Property

A sample of the cable was marked by embossing when the cable came outfrom the extrusion head, i.e. when the cable was not cooled yet.

In order to evaluate the quality of the marking, the sample wasinspected by means of a reflection microscope and a height of about 40μm of the marked letters was measured.

EXAMPLE 5 (COMPARATIVE)

Production of a low-voltage cable similar to that of Example 4 wasundertaken, the only difference being that the external insulating layerof the cable was non-expanded. Therefore, the external insulating layerwas obtained by introducing into the corresponding extruder only thepolymeric composition reported in Table 2 (no master-batch was neededsince no expanding agent was required).

The working conditions of the cable manufacturing process were identicalto those described in Example 4.

Mechanical Properties

A sample of the cable was tested in order to measure the most relevantmechanical properties of the cable.

The ultimate tensile stress, measured according to Standard IEC 608111-1 mentioned above, was of about 16 MPa, while the ultimate elongation,measured according to the Standard mentioned above, was of about of205%.

Electrical Properties

The insulating constant k_(i) was measured according to Italian StandardCEI 20-11 mentioned above and values of about 800 MOhm*km at 20° C. andof about 0.7 MOhm*km at 70° C. were obtained.

Peeling-Off Property

A sample of the cable was tested analogously to the procedure describedin Example 4.

The measured value was of about 0.045 KN.

Marking Property

A sample of the cable was marked by embossing according to the proceduredescribed in Example 4.

The height of the marked letters was of about 20 μm.

1-34. (canceled)
 35. An electric cable comprising a conductor and aninsulating coating surrounding said conductor, said insulting coatinghaving a predetermined thickness and comprising at least two insulatinglayers, said insulating layers, in a radial direction from the insidetoward the outside of said electrical cable, comprising: a. at least oneinsulating layer made of a non-expanded polymeric material and b. atleast one insulating layer made of an expanded polymeric material, saidat least one insulating layer made of an expanded polymeric materialbeing integral with said at least one insulating layer made of anon-expanded polymeric material.
 36. The electric cable according toclaim 35, wherein the thickness of said at least one insulating layermade of a non-expanded polymeric material is at least half of saidpredetermined thickness of said insulating coating.
 37. The electriccable according to claim 36, wherein the thickness of said at least oneinsulating layer made of a non-expanded polymeric material is not lowerthan 70% of said predetermined thickness of said insulating coating. 38.The electric cable according to claim 37, wherein the thickness of saidat least one insulating layer made of a non-expanded polymeric materialis not lower than 85% of said predetermined thickness of said insulatingcoating.
 39. The electric cable according to claim 35, wherein said atleast one insulating layer made of an expanded polymeric material isbonded with said at least one insulating layer made of a non-expandedpolymeric material.
 40. The electric cable according to claim 35,wherein said at least one insulating layer made of an expanded polymericmaterial is co-extruded with said at least one insulating layer made ofa non-expanded polymeric material.
 41. The electric cable according toclaim 35, wherein said at least one insulating layer made a non-expandedpolymeric material adheres to said at least one conductor.
 42. Theelectric cable according to claim 35, wherein said at least oneinsulating layer made of an expanded polymeric material of saidinsulating coating is an intermediate layer between an inner insulatinglayer made of a non-expanded polymeric material and an externalinsulating layer made of an expanded polymeric material.
 43. Theelectric cable according to claim 35, wherein said at least oneinsulating layer made of an expanded polymeric material of saidinsulating coating is an intermediate layer between an inner insulatinglayer made of a non-expanded polymeric material and an externalinsulating layer made of a non-expanded polymeric material.
 44. Theelectric cable according to claim 42 or 43, wherein said intermediateinsulating layer is circumferentially non-continuous in thecross-section.
 45. The electric cable according to claim 44, whereinsaid intermediate insulating layer presents at least one interruption.46. The electric cable according to claim 45, wherein said at least oneinterruption is located along the external profile of said innerinsulating layer.
 47. The electric cable according to claim 45, whereinsaid at least one interruption is located in proximity of the externalprofile of said inner insulating layer.
 48. The electric cable accordingto claim 44, wherein said circumferentially non-continuous intermediateinsulating layer comprises at least one semicircular sector.
 49. Theelectric cable according to claim 48, wherein said at least onesemicircular sector is provided within said inner insulating layer. 50.The electric cable according to claim 48, wherein said at least onesemicircular sector is provided within said external insulating layer.51. The electric cable according to claim 42 or 43, wherein saidintermediate insulating layer is circumferentially continuous in thecross-section.
 52. The electric cable according to claim 35, whereinsaid expanded polymeric material is obtained from a polymeric materialthat, before expansion, has a flexural modulus at room temperature,measured according to ASTM Standard D790, between 20 MPa and 600 MPa.53. The electric cable according to claim 52, wherein said flexuralmodulus is not greater than 200 MPa.
 54. The electric cable according toclaim 53, wherein said flexural modulus is between 20 MPa and 200 MPa.55. The electric cable according to claim 54, wherein said flexuralmodulus is between 10 MPa and 150 MPa.
 56. The electric cable accordingto claim 35, wherein the polymeric material of said at least oneinsulating layer is an expandable polymer selected from the groupcomprising: polyolefins, copolymers of various olefins,olefins/unsaturated esters copolymers, polyesters, and their mixtures.57. The electric cable according to claim 56, wherein said expandablepolymer is polyvinyl chloride.
 58. The electric cable according to claim35, wherein said at least one insulating layer made of a non-expandedpolymeric material and said at least one insulating layer made of anexpanded polymeric material are made of the same base polymericmaterial.
 59. The electric cable according to claim 35, wherein said atleast one insulating layer made of an expanded polymeric material has anexpansion degree between 2% and 500%.
 60. The electric cable accordingto claim 59, wherein said expansion degree is between 5% and 200%. 61.The electric cable according to claim 60, wherein said expansion degreeis between 10% and 50%.
 62. The electric cable according to claim 35,wherein said at least two insulating layers of said insulating coatingpresent an insulating constant (k_(i)) greater than 750 MOhm*km at 20°C.
 63. The electric cable according to claim 35, wherein said at leasttwo insulating layers of said insulating coating present an insulatingconstant (k_(i)) greater than 0.3 MOhm*km at 70° C.
 64. The electriccable according to claim 35, wherein said at least one insulating layermade of an expanded polymeric material has a thickness between 0.05 mmand 1.00 mm.
 65. The electric cable according to claim 64, wherein thethickness of said at least one insulating layer made of an expandedpolymeric material is between 0.10 mm and 0.50 mm.
 66. A process formanufacturing an electric cable, said cable comprising a conductor andan insulating coating surrounding said conductor and comprising, in aradial direction from the inside toward the outside of said electricalcable, at least one insulating layer made of a non-expanded polymericmaterial and at least one insulating layer made of an expanded polymericmaterial, comprising the steps of: feeding said conductor to anextruding machine; depositing by co-extrusion: a non-expandablepolymeric material in a position radially external to said conductor soas to form said at least one insulating layer made of a non-expandedpolymeric material; and an expandable polymeric material in a positionradially external to said at least one insulating layer made of anon-expanded polymeric material so as to form said at least oneinsulating layer made of an expanded polymeric material; and expandingsaid expandable polymeric material during said step of depositing byco-extrusion.
 67. The process according to claim 66, wherein said stepof expanding is effected during said step of depositing by co-extrusionby adding an expanding agent.
 68. The process according to claim 67,wherein said step of expanding is effected during said step ofdepositing by co-extrusion by injecting a gas at a high pressure.